Feb. 25, 2015

The following are two excerpts from the book This Idea Must Die, edited by John Brockman. Listen to SciFri on Friday, February 27, 2015, to hear Seth Lloyd and Sean Carroll discuss their ideas. And let us know if you think these scientific ideas must die by clicking the Twitter links after each essay.

I know. The universe has been around for 13.8 billion years and is likely to survive for another 100 billion years or more. Plus, where would the universe retire to? Florida isn’t big enough. But it’s time to retire the 2,500-year-old scientific idea of the universe as the single volume of space and time that contains everything. Twenty-first-century cosmology strongly suggests that what we see in the cosmos—stars, galaxies, space and time since the Big Bang—does not encompass all of reality. Cosmos, buy the condo.

What is the universe, anyway? To test your knowledge of the universe, please complete the following sentence. The universe

(a) consists of all things visible and invisible—what is, has been, and will be.

(b) began 13.8 billion years ago in a giant explosion called the Big Bang and encompasses all planets, stars, galaxies, space, and time.

(c) was licked out of the salty rim of the primordial fiery pit by the tongue of a giant cow.

(d) All of the above.

(Correct answer below.)

The idea of the universe as an observed and measured thing has persisted for thousands of years. Those observations and measurements have been so successful that today we know more about the origin of the universe than we do about the origin of life on Earth. But the success of observational cosmology has brought us to a point where it’s no longer possible to identify the universe—in the sense of answer (a) above—with the observed cosmos—answer (b). The same observations that establish the detailed history of the universe imply that the observed cosmos is a vanishingly small fraction of an infinite universe. The finite amount of time since the Big Bang means that our observations extend only a little more than 10 billion light-years from Earth. Beyond the horizon of our observation lies more of the same—space filled with galaxies stretching on forever. No matter how long the universe exists, we will have access to only a finite part, while an infinite amount of universe remains beyond our ken. All but an infinitesimal fraction of the universe is unknowable.

That’s a blow. The scientific concept universe = observable universe has thrown in the towel. Perhaps that’s OK. What’s not to like about a universe encompassing infinite unknowable space? But the hits keep coming. As cosmologists delve deeper into the past, they find more and more clues that, for better or worse, there’s more out there than just the infinite space beyond our horizon. Extrapolating backward in time to the Big Bang, cosmologists have identified an epoch called inflation, in which the universe doubled in size many times over a tiny fraction of a second. The vast majority of spacetime consists of this rapidly expanding stuff. Our own universe, infinite as it is, is just a “bubble” that has nucleated in this inflationary sea.

It gets worse. The inflationary sea contains an infinity of other bubbles, each an infinite universe in its own right. In different bubbles, the laws of physics can take different forms. Somewhere out there in another bubble universe, the electron has a different mass. In another bubble, electrons don’t exist. Because it consists not of one cosmos but of many, the multibubble universe is often called a multiverse. The promiscuous nature of the multiverse may be unappealing (William James, who coined the word, called the multiverse a “harlot”), but it’s hard to eliminate. As a final insult to unity, the laws of quantum mechanics indicate that the universe is continually splitting into multiple histories, or “many worlds,” out of which the world we experience is only one. The other worlds contain the events that didn’t happen in our world.

After a two-millenium run, the universe as observable cosmos is kaput. Beyond what we can see, an infinite array of galaxies exists. Beyond that infinite array, an infinite number of bubble universes bounce and pop in the inflationary sea. Closer by, but utterly inaccessible, the many worlds of quantum mechanics branch and propagate. MIT cosmologist Max Tegmark calls these three kinds of proliferating realities the type I, type II, and type III multiverses. Where will it all end? Somehow, a single, accessible universe seemed more dignified.

There’s hope, however. Multiplicity itself represents a kind of unity. We now know that the universe contains more things than we can ever see, hear, or touch. Rather than regarding the multiplicity of physical realities as a problem, let’s take it as an opportunity.

Suppose that everything that could exist does exist. The multiverse is not a bug but a feature. We have to be careful: The set of everything that could exist belongs to the realm of metaphysics rather than physics. Tegmark and I have shown that with a minor restriction, however, we can pull back from the metaphysical edge. Suppose that the physical multiverse contains all things that are locally finite, in the sense that any finite piece of the thing can be described by a finite amount of information. The set of locally finite things is mathematically well defined: It consists of things whose behavior can be simulated on a computer (more specifically, on a quantum computer). Because they’re locally finite, the universe we observe and the various other universes are all contained within this computational universe. As is, so somewhere, a giant cow.

In a world where scientific theories often sound bizarre and counter to everyday intuition and a wide variety of nonsense aspires to be recognized as “scientific,” it’s important to be able to separate science from non-science—what philosophers call “the demarcation problem.” Karl Popper famously suggested the criterion of “falsifiability”: A theory is scientific if it makes clear predictions that can be unambiguously falsified.

It’s a well-meaning idea but far from the complete story. Popper was concerned with theories such as Freudian psychoanalysis and Marxist economics, which he considered non-scientific. No matter what actually happens to people or societies, Popper claimed, theories like these will always be able to tell a story in which the data are compatible with the theoretical framework. He contrasted this with Einstein’s relativity, which made specific quantitative predictions ahead of time. (One prediction of general relativity was that the universe should be expanding or contracting, leading Einstein to modify the theory because he thought the universe was actually static. So even in this example, the falsifiability criterion is not as unambiguous as it seems.)

Modern physics stretches into realms far removed from everyday experience, and sometimes the connection to experiment becomes tenuous at best. String theory and other approaches to quantum gravity involve phenomena that are likely to manifest themselves only at energies enormously higher than anything we have access to here on Earth. The cosmological multiverse and the many-worlds interpretation of quantum mechanics posit other realms impossible for us to access directly. Some scientists, leaning on Popper, have suggested that these theories are non-scientific because they’re not falsifiable.

The truth is the opposite. Whether or not we can observe them directly, the entities involved in these theories are either real or they are not. Refusing to contemplate their possible existence on the grounds of some a-priori principle, even though they might play a crucial role in how the world works, is as non-scientific as it gets.

The falsifiability criterion gestures toward something true and important about science, but it’s a blunt instrument in a situation calling for subtlety and precision. It’s better to emphasize two central features of good scientific theories: They’re definite and they’re empirical. By “definite,” we mean that they say something clear and unambiguous about how reality functions. String theory says that in certain regions of parameter space, ordinary particles behave as loops or segments of one-dimensional strings. The relevant parameter space might be inaccessible to us, but it’s part of the theory which cannot be avoided. In the cosmological multiverse, regions unlike our own are unambiguously there even if we can’t reach them. This is what distinguishes these theories from the approaches Popper was trying to classify as non-scientific. (Popper himself understood that theories should be falsifiable “in principle,” but that modifier is often forgotten in contemporary discussions.)

It’s the “empirical” criterion that requires some care. On the face of it, this criterion might be mistaken for “makes falsifiable predictions.” But in the real world, the interplay between theory and experiment isn’t so cut and dried. A scientific theory is ultimately judged by its ability to account for the data—but the steps along the way to that accounting can be indirect.

Consider the multiverse, often invoked as a potential solution to some of the fine-tuning problems of contemporary cosmology. For example, we believe there’s a small but nonzero vacuum energy inherent in empty space. This is the leading theory to explain the observed acceleration of the universe, for which the 2011 Nobel Prize in physics was awarded. The problem for theorists is not that vacuum energy is hard to explain; it’s that the predicted value is enormously larger than what we observe.

If the universe we see around us is the only one there is, the vacuum energy is a unique constant of nature and we’re faced with the problem of explaining it. If, on the other hand, we live in a multiverse, the vacuum energy could be completely different in different regions, and an explanation suggests itself immediately: In regions where the vacuum energy is much larger, conditions are inhospitable to the existence of life. There is therefore a selection effect, and we should predict a small value of the vacuum energy. Indeed, using this precise reasoning, Steven Weinberg did predict the value of the vacuum energy long before the acceleration of the universe was discovered.

We can’t (as far as we know) observe other parts of the multiverse directly, but their existence has a dramatic effect on how we account for the data in the part of the multiverse we do observe. It’s in that sense that the success or failure of the idea is ultimately empirical: Its virtue is not that it’s a neat idea, or fulfills some nebulous principle of reasoning, but that it helps us account for the data. Even if we’ll never visit those other universes.

Science isn’t merely armchair theorizing, it’s about explaining the world we see, developing models that fit the data. But fitting models to data is a complex and multifaceted process, involving a give-and-take between theory and experiment, as well as the gradual development of theoretical understanding in its own right. In complicated situations, fortune-cookie-sized mottos like “Theories should be falsifiable” are no substitute for careful thinking about how science works. Fortunately, science marches on, largely heedless of amateur philosophizing. If string theory and multiverse theories help us understand the world, they’ll grow in acceptance. If they prove ultimately too nebulous, or better theories come along, they’ll be discarded. The process might be messy, but nature is the ultimate guide.

About John Brockman

The founder and publisher of the influential online science salon Edge.org, John Brockman is the editor of The Universe, This Explains Everything, and other books. He is the CEO of the literary agency Brockman Inc., and lives in New York City.

The views expressed are those of the author and are not necessarily those of Science Friday.